The present invention relates to an antenna for radiating and receiving circular polarized electromagnetic signals in particular signals with microwave or mm-wave frequencies.
Such antennas are of particular interest for high data rate applications. Typical applications of that type are satellite-earth-communication, indoor LOS, wireless LANS or outdoor LOS private links. These applications require large bandwidths which can only be granted in very high frequency regions as e.g. from 15 GHz to 60 GHz. The circular polarization is necessary in order to omit the requirement for the user to observe the orientation of the antenna.
Antennas for the use in this field of application have been described in the prior art. In EP 0 481 048 B1 for example a microstrip radiator for circular polarization is described. This antenna comprises a substrate on which a radiating patch element is located, the patch element is separated from a conductive layer surrounding the patch by a slot and is connected to the surrounding conductive layer by a spiral band. An input strip is provided on the opposite side of the substrate. In this antenna the radiating element is the path and the wire like spiral band. The slot is not considered to be a radiating element.
The object of the present invention is to provide an antenna which allows applications into the mm wave frequencies within a very large band width with good efficiency, which has a high gain and is simple in structure.
This object is achieved by an antenna comprising, a planar dielectric substrate comprising a front and a back dielectric face, at least one subantenna means comprising a first and second element for radiating and receiving circular polarized electromagnetic signals, at least one transmission line means for transmitting signals from and to said at least one subantenna means, characterized in that the first and second elements of the subantenna means are slots of a spiral shape and are connected to each other to form a double spiral slot with both spirals having the same sense of rotation, the subantenna means being arranged on the front dielectric face of the substrate and the transmission line means being arranged on the back dielectric face of the substrate.
The main advantages of the antenna according to the present invention are its simple structure and the decoupling of the feed network from the radiating structure. Finally with the antenna according to the present invention a higher gain than with other types of antennas can be achieved.
The simplicity of the structure is given by the fact that the feed line and the subantenna means are both formed on one single dielectric substrate on opposite sides thereof. For the inventive arrangement, hence, already a single layer substrate suffices. An additional alignment of a path on an upper layer is therefore not required. Such alignments are mandatory for aperture coupled patch path antennas. The tolerance is very small for high frequencies and therefore such an alignment is a tedious task. The possibility of omitting such an alignment during manufacturing of the antenna allows the use of cheaper technology and thereby decreases the overall costs. The simple structure and low costs are a strong necessity for a commercial success of an antenna and are met by the inventive structure.
According to the present invention the subantenna means and the transmission line are arranged on a dielectric substrate, which preferably has a dielectric constant of xcex51xe2x89xa71. Suitable material for the dielectric substrate is for example Teflon-fiberglass with a dielectric constant of 2.17. The subantenna means are dual-spiral slot which are preferably formed in a metal coated area on one of the faces of the dielectric substrate. They can be obtained by metallizing one side of the substrate and etching the slots into the metallic layer by known etching techniques. The feed structure is obtained by applying metal to the opposite side of the substrate in the desired shape. Both the feed structure and the slots can be produced by using well known photo etching or thick film processing or the like.
With the feed line, which may have an additional feed network, being arranged on the opposite side of the substrate from the subantenna means it is ensured that the feed structure is decoupled from the radiating spiral slots and that radiation of the antenna is only determined by the subantenna means, namely the radiating spiral slots, which are well controllable.
The shape of the radiating elements of the subantenna means is that of a spiral. The spiral slots are arranged such that the outer end of one spiral is connected to the outer end of the other spiral, so that a long slot is formed. By arranging the sense of rotation of the two spirals to be equal an even radiation pattern of the antenna can be achieved.
The length of the dual-spiral slot is crucial to the operation. It is normally chosen in order to allow for a traveling wave, which radiates while traveling from the connection of the two spirals, which is as will be described later, to the end of the spiral. The length should be chosen such that the average current density is constant through out the slot.
Further advantageous features of the antenna according to the present invention are defined in the subclaims.
The spirals which form the slots for radiating the signals can have less than one, one or multiple spiral turns. The choice of the geometry of the spiral slots offers the possibility to vary the length of radiating slots to be formed. Also the area covered by the subantenna means and thereby the overall antenna size can be adjusted by the appropriate choice. It is not necessary to design the spiral slots such that the spiral is open at the end. The antenna according to the present invention can also be operated with a subantenna means comprising two closed loops. Furthermore not each of the spirals has to be formed by a slot bent to the desired rounded shape. The open or closed spiral can also be realized by a rhombic like structure. In the later case the turns of the spiral will not be smooth but will have edges.
Elements of the subantenna means of any of the described shapes are attached to each other to form a long slot. The connection between the two spiral slots can be achieved by simple attaching the end of the spiral slot that extends outwards to the respective end of the other slot. In a preferred embodiment the first and second element of the subantenna means are, however, connected via a small linear coupling section. This section also is formed by a slot. The angle between the coupling slot and the beginning of the spiral slots can be within the range of 0 to 90xc2x0. If two open spirals are connected via a straight coupling section under an angle of 0xc2x0 the overall slot results in an S-shape. The spirals are always connected to each other in a way as to avoid an overlapping of the two spirals. Furthermore the spiral slots must not be arranged in such a way that a part of the turn of the spiral or a whole turn crosses the projection of the feed line, which is arranged on the opposite side of the substrate. Such an overlap would result in an unwanted coupling.
The coupling slot in contrast has to be positioned such that the projection thereof crosses the feed line. Also if no straight coupling slot between the two spirals is provided the coupling of the dual spiral slot to the feed line preferably takes place close to or at the center of the slot. In a preferred embodiment the section of the dual spiral slot, where the feed line and the subantenna means overlap in their projection, is perpendicular to the direction of the feed line. This way an equal distribution of power between the two spirals can be expected which leads to an even radiation pattern of the antenna.
If the coupling section of the dual-spiral slot is not perpendicular to the feed line direction the power will be unevenly distributed between the two spirals. In that case the electric field strength in one of the spirals is much higher than in the other spiral. This phenomenon can be compensated by adjusting the length of the respective spiral slots. The antenna of the present invention can therefore also be designed such that the first or the second element of the subantenna means is greater in length than the other. The spiral with the shorter length would then be used as the spiral at the side where the higher power supply occurs due to the position of the coupling section of the dual-spiral slot.
Another way of compensation for the uneven power distribution is by designing the antenna geometry such that the two spirals have different radii. The smaller the radius of the spiral, the smaller the electric field strength in the spiral slot will be.
It is also within the scope of the invention that the spirals have a varying radius over the turn. Finally the slot forming the spirals can be of an even width over the length or the width can increase in each of the two spirals forming the dual-spiral slot. In the latter case the width preferably increases from the coupling section to the end of each slot.
Besides the possibility of compensating unequal power distribution between the two spirals it has to be noted that the length of each spiral and the radius of its turn must be adjusted appropriately in order to allow the forming of a circular polarized wave. In the antenna according to the invention the exciting wave is guided to the slot area through the microstrip feed line. Here the magnetic field component excites an electric field within the slot. The length and radius have to be chosen such that a phase difference of 90xc2x0 is introduced between the magnetic field component at the vertical and the horizontal parts of the spirals. This leads to a phase shift of 90xc2x0 between the radiated vertical and the horizontal electric field components. Due to this phase shift a circular polarized radiation at the correct frequency of operation can be obtained.
The antenna according to the present inventions can advantageously further comprise a reflector means. This reflector means which is normally represented by a reflector plate is spaced to and parallel with the back face of the dielectric substrate. Between said reflector means or plate and said back face of the substrate, low loss material should be located. Even though the inventive antenna can be operated without any reflector means, such means can be added in order to enlarge the broadside gain of the antenna and to cancel the backside radiation. The reflector plate is preferably spaced from the middle of the substrate on which the feed line is located in a distance of xc2xc xcex, wherein xcex is the center frequency of operation.
The inventive antenna is in particular suitable for being arranged as an antenna element in a phase antenna array comprising a plurality of antenna elements. The planar phase antenna array can be obtained by arranging several subantenna means each including a dual-spiral slot on one substrate and feeding this arrangement by means of a feeding network, located on the opposite face of the substrate. In such an arrangement the advantage of the present invention specifically come to fruition. The arranging of the feed line on the opposite side of the substrate from the subantenna means provides a possibility of decoupling of the feed network from the radiating structure. With conventional antennas, in particular in array configuration, spurious unwanted radiation components are observed from the feed network. These components greatly decrease the axial ratio and are therefore undesirable. In the antenna according to the present invention in contrast the feeding network is completely decoupled from the subantenna means and thus the radiation is only determined by the well controllable subantenna means, namely the radiating dual-spiral slots.
In a preferred embodiment the feeding network for such a planar phase antenna array is realized by tapered microstrips. By the use of tapered microstrips no quarter wave transformers have to be used.